Amazon Underground is a standalone app store for Android smartphones and tablets. As the name suggests, the app comes from Amazon and offers unique benefits to users. Every app contained within Underground is downloadable free of charge. Even in-app purchases are available for free, which is unheard of with today's mobile apps. By partnering with app makers, Amazon has created perhaps the most interesting app store today.

Most Android devices are compatible with Amazon Underground. Users are ready to get started as soon as they sign into an existing Amazon account. At this point, they're presented with hundreds upon hundreds of mobile apps. Apps range from games to productivity tools and more. 100% of these Android apps are free to download, and in-app purchases that normally cost money cost nothing.

Amazon allows app makers to track users' interactions and usage within each app. For this capability, app makers agree to give Amazon Underground users everything their apps offer for free. It's a unique partnership between Amazon, developers, and users. Smartphone and tablet owners benefit from this relationship in countless ways. In most cases, premium apps are better than free alternatives, but not everyone is willing to pay for apps.

In the end, Amazon Underground is quite useful for the average mobile device owner. All features of a given app are unlocked immediately and without charge. Not all apps in Underground are noteworthy, but many apps are worth a download, especially for free. Fortunately, Amazon Underground is populated with many well-reviewed apps, and the Underground app itself runs without issue.